The driving force transmission path of a wheel loader from the engine to the tires is provided with a transfer, a modulation clutch, a torque converter, a transmission having a forward clutch, a reverse clutch and each speed clutch, and an axle.
The engine output (torque) is transmitted to the tires via the transfer, the modulation clutch, the torque converter, the transmission, and the axle.
Conventionally, in a working vehicle such as a wheel loader, the modulation clutch is provided between the engine and the torque converter, and is provided to vary the degree of engagement between an input side and an output side to vary the driving force transmitted from the engine to the torque converter. The modulation clutch is constituted as a hydraulic clutch, in which by varying hydraulic pressure (hereafter referred to as the modulation clutch pressure), the hydraulic clutch is connected (engaged) or disconnected (disengaged).
A wheel loader mainly carries out its work in a V-shaped operation. The V-shaped operation means the operation in which the wheel loader advances into a mound of soil, reverses after excavating the soil, goes forward when the wheel loader arrives at direction changing point and loads the soil into a hopper or a dump truck, and repeats this operation.
When excavating, the wheel loader forces an excavation work equipment into the mound of soil while accelerating. At this time a load on the tires is large, and a large driving force is transmitted from the engine to the tires.
Therefore, when excavating in this way, the driving force transmitted to the tires is increased, and frequently tire slip occurs between the tires and a ground surface. If the ground surface is not uniform, the load on each tire will differ, so slip will occur at one of the four tires. Further, at the time of forcing into the mound of soil, slip relative to the ground, in other words slip of both wheels, occurs very little. Tire slip occurs at only one of the left or right wheels, and the other drive wheel is static.
Tire slip not only damages work efficiency of a wheel loader, but it also causes major and serious damage to the expensive large diameter tires for working vehicles, such as cuts and perforation of the tread, and so on. Also, even if major and serious damage is not caused, progress of wear and damage to the tires is faster. Therefore, durability of the tires is reduced, and costs for changing tires are serious for users. Therefore, it is necessary to prevent tire slip.
Therefore, conventionally when operating the wheel loader, tire slip is prevented by controlling degree of engagement of the modulation clutch.
Patent Document 1 discloses an invention in which a difference in rotational speeds of left and right drive wheels of a working vehicle is calculated, and as the difference in calculated rotational speed increases, a modulation clutch pressure is decreased to weaken degree of engagement of the modulation clutch, and reduce a driving force transmitted to the tires.
Patent Document 2 discloses an invention in which slip is measured based on a difference in rotational speed of front left and right drive wheels and a difference in rotational speed of rear left and right drive wheels in a 4-wheel drive working vehicle, and if slip is detected, a hydraulic clutch of a transmission is put into a half clutch state, and a driving force transmitted to the tires is reduced.
Patent Document 3 discloses an invention in which it is determined whether excavation is being carried out by measuring a work equipment position and work equipment hydraulic pressure, and only in this case, slip control is carried out.
Patent Document 4 discloses an invention in which tire slip is calculated based on an engine rotation speed and a transmission output rotation speed, and if tire slip is detected, an engine output characteristic is switched to a low torque characteristic, to reduce a driving force transmitted to the tires.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-146928
Patent Document 2: Japanese Patent Application Laid-open No. 6-58345
Patent Document 3: Japanese Patent Application Laid-open No. 2005-146886
Patent Document 4: Japanese Patent Application Laid-open No. 2001-115870
The inventions according to Patent Documents 1, 2, 4 each infer whether tire slip has occurred from the rotational speed state of parts on the transmission path from the engine to the tires (drive wheels) or the vehicle speed, and when it is inferred that tire slip has occurred, control is carried out so as to reduce the tire slip, such as by decreasing the modulation clutch pressure, or changing the engine characteristics. Therefore, when it is inferred that tire slip is not occurring, the control to reduce the tire slip is canceled, and control is carried out to increase the modulation clutch pressure to restore the normal modulation clutch pressure state, or to restore the characteristics to the normal engine characteristics.
However, tire slip is a phenomenon in which the driving force transmitted to the tires exceeds an adhesion limit between the tires and the ground surface, in other words exceeds the maximum static friction force, so the tires rotate without the vehicle moving. Also, once the tires start slipping, the tires receive friction from the ground surface with dynamic friction force, which is lower than the adhesion limit. As a result, the reactive force from the ground surface is dramatically reduced and whereby the minimum driving force that requires to cause the tire slip is reduced.
In this way there is hysteresis in the driving force causing tire slip, so if the inventions according to Patent Documents 1, 2, 4 are applied as they are, tire slip will occur again due to the process of carrying out the control to restore the modulation clutch pressure or the like to the normal state, even though tire slip will be temporarily stopped by the control to suppress the tire slip. Subsequently hunting occurs in which the tire slip suppression state (tire non-slip state) and the tire slip state are repeated alternately.
In other words, taking the invention according to Patent Document 1 as an example, when it is inferred that tire slip has occurred, the tire slip is temporarily suppressed by decreasing the modulation clutch pressure to weaken the degree of engagement of the modulation clutch to reduce the driving force transmitted to the tires. Although the tire slip is temporarily suppressed, after suppression of the tire slip the modulation clutch pressure is gradually increased in accordance with the difference in rotational speed of the left and right drive wheels only, without consideration of the driving force transmitted to the tires or the dynamic friction force. Therefore, tire slip occurs again while increasing the modulation clutch pressure. Thereafter the tire slip suppression by decreasing the modulation clutch pressure and occurrence of tire slip by increasing the modulation clutch pressure are repeated alternately.
Therefore in the conventional technique, although tire slip is temporarily suppressed, tire slip repeatedly occurs throughout the whole period that excavation is being carried out. This is an equivalent to having tire slip occurring continuously during excavation operation.
Therefore, the problem of reduction of work efficiency due to tire slip, and the problem of reduction in durability due to tire damage and the large cost for the user of changing the tires cannot be solved with the conventional technique.
Also, the invention according to Patent Document 3 discloses a technique for determining whether slip has occurred in the case where it is judged that excavation operation is being carried out. However, this does not mention slip control specifically.
With the foregoing in view, the present invention solves the problem of reduction of work efficiency due to tire slip and the problems of reduction in durability due to tire damage and the large cost for the user of changing the tires, due to repeated occurrence of tire slip in situations where there is the possibility of occurrence of tire slip, such as during excavation and the like.